This invention relates generally to miniature circuit breaker devices and more particularly to such devices which are adapted to switch electric loads, as well as provide overcurrent protection for such loads.
One of the primary uses of circuit breakers of the type with which the invention relates is to switch and protect electrical loads in aircraft. To be acceptable for this purpose, such circuit breakers need to be small in size yet highly reliable. Such devices are manually actuatable, as well as being responsive to open circuits upon current overloads.
One such circuit breaker which has found wide acceptance is disclosed and claimed in U.S. Pat. No. 3,361,882. In accordance with this patent, the circuit breaker includes first current carrying and second ambient compensating thermostatic elements mounted in a casing, each having an end free for movement, with a slide adapted to transfer motion from the free end of the first element to the free end of the second element. A catch is attached to the second thermostatic element and movable therewith and is adapted to cooperate with a first latch rotatably mounted at the end of a plunger. A bridging movable contact is carried by the latch and is moved thereby to engage and disengage a pair of stationary contacts. A second latch engages the plunger to maintain the plunger in a position with the contact in engagement and is releasable to permit the plunger under the influence of a return spring to move toward an open contacts position when the first latch is released from the catch. The free ends of the bimetallic elements move the same amount under varying ambient temperatures to maintain the same relative position of the first latch and the catch.
Although the above described circuit breaker is very effective and has a life expectancy exceeding 2500 cycles, it has become desirable to provide a device which has even greater life expectancy, as well as one which has a wider current carrying range of ratings and improved immunity to vibration.
For example, in the above referenced circuit breaker, one of the limiting factors regarding useful product life is the fact that a ball latch mechanism is used to maintain the contacts in the closed position. Due to the fact that the spherical elements experience wear, they thereby affect calibration of the device and deleteriously limit its longevity. Further, the catch and first latch are subjected to high frictional forces which results in changes in the calibrated current levels for tripping the breaker over the life of the device.